Posts By: Judit Cabana Dominguez, PhD

Our body is colonized by trillions of microorganisms that are important for vital processes. Gut microbiota are the microorganisms living in the intestinal gut and play an essential role in digestion, vitamin synthesis and metabolism, among others. The mouth and the large intestine contain the vast majority of gut microbiota whether the stomach only contains few thousands of microorganisms, especially due to the acidity of its fluids. Microbiota composition is constantly changing, affecting the well-being and health of the individual.

Each individual has a unique microbiota composition, and it depends on several factors including diet, diseases, medication and also the genetics of the individual (host) (Figure). Some medicines, especially antibiotics, reduce bacterial diversity. Strong and broad spectrum antibiotics can have longer effects on gut microbiota, some of them up to several years. Genetic variation of an individual also affects the microbiota composition, and the abundance of certain microorganisms is partly genetically determined by the host.

The main contributor to gut microbiota diversity is diet, accounting for 57% of variation. Several studies have demonstrated that diet’s composition has a direct impact on gut microbiota. For example, an study performed on mice showed that “Western diet” (high-fat and sugar diet), alters the composition of microbiota in just one day! On the other hand, vegetarian and calorie restricted diet can also have an effect on gut microbiota composition.

Prebiotics and probiotics are diet strategies more used to control and reestablish the gut microbiota and improve the individual’s health. Probiotics are non-pathogenic microorganisms used as food ingredients (e.g. lactobacillus present in yoghurt) and prebiotics are indigestible food material (e.g. fibers in raw garlic, asparagus and onions), which are nutrients to increase the growth of beneficial microorganisms.

In the last years the new term psychobiotics has been introduced to define live bacteria with beneficial effects on mental health. Psychobiotics are of particular interest for improving the symptomatology of psychiatric disorders and recent preclinical trials have show promising results, particularly in stress, anxiety and depression.

Overall, these approaches are appealing because they can be introduced in food and drink and therefore provide a relatively non-invasive method of manipulating the microbiota.

The human gut is colonized by microorganisms in a similar number as the cells of the human body.

“Microbiota” refers to these microorganisms, and it maintains a symbiotic relationship with the host, contributing to essential functions such as food digestion, energy harvest and storage, the function of the intestinal barrier, and the immune system and protection against pathogenic organisms. Prenatal and postnatal factors can alter the composition of the microbiota, such as stress and diet or the use of antibiotics (see image).

For instance, stress during pregnancy can alter the composition of vaginal microbiota, which affects the composition of the microbiota of the newborn and is related to gastrointestinal (GI) symptoms and allergic reactions. Interestingly, there is a bidirectional communication between the GI tract and the central nervous system (the gut-brain axis) that involves neuronal and metabolic pathways, immune and endocrine mechanisms. Changes in the composition of the microbiota can lead to altered development of the brain and increased risk of psychiatric and neurodevelopmental disorders, such as anxiety, depression and autism (see image).

Depression is one of the most recurrent stress-related disorders that highly impacts the quality of life. Fecal samples of patients with depression have a decreased microbial richness and diversity than controls. The use of probiotics have been shown to help with sad mood and negative thoughts, which may be a potential preventive strategy for depression.

Autism is characterized by impaired communication, poor social engagement and repetitive behaviours, with frequent GI symptoms. We know that the bacteria composition is more diverse in autistic individuals than in unaffected subjects.

For other psychiatric disorders, such as Attention deficit/hyperactivity disorder (ADHD) and Schizophrenia, there is indirect evidence for a role of the microbiota, but more studies are needed.

This connection between the gut and brain is two way communication, and is known as “The Gut-Brain Axis.”

Our knowledge of the impact of gut microbiota on brain function is growing fast, which may pave the way to possible applications for the treatment of psychiatric and neurodevelopmental disorders.

Authors Judit Cabana, Bru Cormand, and Noelia Fernandez Castillo are in the Department of Genetics, Microbiology &amp; Statistics, University of Barcelona, Catalonia, Spain

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